This invention relates to radiation-based inspection techniques, and more particularly to interferometric profilometry systems and methods which may be used to measure the shape, thickness, and material inhomogeneity of a wafer.
Phase shifting interferometry (PSI) is a highly accurate and efficient phase measuring method applied to a variety of applications including optical testing, surface profilometry, surface roughness estimation, and surface displacement measurement. The fundamental concept of PSI is that the phase of an interferogram can be extracted accurately by acquiring a set of phase-shifted interferograms. The phase shifts between interferograms are produced by changing the optical path difference (OPD) between the measurement surface and a reference surface. The phase shifts also can be achieved by changing the wavelength of the radiation used, if the OPD between the measurement surface and the reference surface is not zero.